(1) Field of the Invention
This invention relates to a method for forming a ferroelectric capacitor and a method for fabricating a semiconductor device and, more particularly, to a method for forming a ferroelectric capacitor in which a ferroelectric material is used for forming a dielectric layer and a method for fabricating a semiconductor device having such a ferroelectric capacitor.
(2) Description of the Related Art
The polarization state of a ferroelectric material can be reversed by an external electric field. Nonvolatile semiconductor memories in which this characteristic is utilized, that is to say, ferroelectric random access memories (FeRAMs) have conventionally been developed and manufactured.
As with dynamic random access memories (DRAMs), each memory cell in FeRAMs includes a switching transistor and a capacitor. A ferroelectric material is used for forming a dielectric layer in this capacitor. For example, lead zirconate titanate (Pb(Zr,Ti)O3), or PZT, is used as such a ferroelectric material.
A ferroelectric capacitor included in such an FeRAM is obtained by forming a lower electrode layer, a layer of a ferroelectric material, and an upper electrode layer by, for example, a sputtering method. In this case, annealing treatment is performed under predetermined conditions at the time of forming the layer of a ferroelectric material in order to crystallize the ferroelectric material which is formed as a film and which is in an amorphous state (see Japanese Patent Laid-Open Publication Nos. 2001-126955 and 2002-246564).
By the way, the polarization axis of the tetragonal PZT orients in the (001) direction. Therefore, the polarization value of the tetragonal PZT is greatest when it is oriented in the c-axis direction. Usually, however, it is difficult to form PZT oriented in the c-axis direction on a polycrystalline substrate.
Accordingly, to form a ferroelectric capacitor included in a FeRAM of PZT, platinum (Pt) the lattice constant of which is comparatively close to that of PZT is used first for forming a lower electrode layer. A Pt film formed by, for example, a DC sputtering method is apt to be oriented in the direction of the (111) plane, being the densest plane. A PZT layer is then formed on the Pt film so that the (111) plane will preferentially be oriented. If the PZT layer is formed in this way, a switching (polarization inversion) direction forms an angle of 45° with an inverted electric field. The polarization value of the PZT layer formed in this way is smaller than the polarization value of a PZT layer the (001) plane of which is preferentially oriented. However, the polarization value of the PZT layer formed in this way is comparatively great and this PZT layer can adequately be utilized as a nonvolatile memory. As a result, an FeRAM which has good characteristics and which is suitable for production can be fabricated.
For example, an RF sputtering method is used for forming the PZT layer on the lower electrode layer formed of platinum. The PZT layer immediately after the sputtering is in an amorphous state, so it is then crystallized by annealing treatment. In a conventional method for fabricating an FeRAM, annealing conditions are properly selected at the time of forming the PZT layer in this way so that the (111) plane of the PZT layer can preferentially be orientated.
To crystallize the formed PZT layer which is in an amorphous state, usually rapid thermal anneal (RTA) is performed in an atmosphere in which an oxidation gas, such as oxygen (O2) exists by using a lamp annealer or the like. In this case, the following method has conventionally been adopted. After an wafer on which the PZT layer is formed is transported into a predetermined heat treatment system, air which flows into the heat treatment system with the wafer at transportation time is purged by passing a certain amount of O2 gas and a large amount of a non-oxidation gas, such as an inert gas, in the heat treatment system. After the elapse of a certain period of time, the flow rate of the inert gas is reduced to set gas composition and a gas flow rate at the time of raising the temperature. Immediately after the gas composition and the gas flow rate are set, the raising of the temperature is begun. For example, a certain amount of O2 gas and a large amount of argon (Ar) gas are used at the time of purging the air, and a mixed gas that contains the certain amount of O2 gas and the Ar gas the flow rate of which is reduced is used at the time of raising the temperature.
FIG. 8 is a view for describing an example of a conventional crystallization annealing method. In FIG. 8, a horizontal axis indicates time and a vertical axis indicates a gas flow rate and temperature.
As shown in FIG. 8, when annealing treatment is performed for crystallizing the PZT layer, the wafer is transported into the heat treatment system. To purge the air which flows into the heat treatment system, a certain amount of O2 gas and a large amount of Ar gas are introduced first into the heat treatment system at the same time. The flow rate of the O2 gas is controlled by a mass flow controller, being a flow rate controller, and the flow rate of the Ar gas is controlled by another mass flow controller, being a flow rate controller. When the air is purged, the flow rate of the O2 gas has already been set to a flow rate value required later at the time of raising and maintaining the temperature by using the mass flow controller. After the elapse of a certain period of time, the mass flow controller for Ar gas is adjusted to reduce the flow rate of the Ar gas. By doing so, a mixed gas several percent of which is the O2 gas and the rest of which is the Ar gas is introduced into the heat treatment system. Immediately after that, the raising of the temperature is begun.
However, if this method is used, the composition (partial pressure) or flow rate of the mixed gas which contains the O2 gas and the Ar gas may not be a target value at the time of beginning raising the temperature. Accordingly, even if crystallization annealing treatment is performed under the same conditions, PZT crystals having stable characteristics may not be obtained.
FIG. 9 shows an example of variation in the orientation rate of PZT obtained by using the conventional crystallization annealing method. In FIG. 9, a horizontal axis indicates a sample wafer number and a vertical axis indicates the orientation rate (%) of the (222) plane of the PZT layer.
After the crystallization annealing treatment, measurements are made by utilizing X-ray diffraction (XRD). Integrated intensity for the (101), (100), and (222) planes of the PZT layer is found from their diffraction peaks and the orientation rate of the (222) plane of the PZT layer is calculated by
orientation rate (%) of (222) plane of PZT layer={integrated intensity for (222) plane of PZT layer}×100/[{integrated intensity for (100) plane of PZT layer}+{integrated intensity for (101) plane of PZT layer}+{integrated intensity for (222) plane of PZT layer}]
In FIG. 9, conditions under which crystallization annealing treatment is performed on each sample are the same.
As stated above, when the (001) plane of the PZT layer is preferentially orientated, its polarization value is greatest. From the viewpoint of FeRAM production, however, it is preferable that the (111) plane ((222) plane) should be preferentially oriented. If the (111) plane ((222) plane) is preferentially oriented, a switching direction forms an angle of 45° with an inverted electric field and a comparatively great polarization value is obtained. As can be seen from FIG. 9, if crystallization annealing treatment is performed under the same conditions, the orientation rate of the (222) plane of the PZT layer is 90% or greater for all samples. However, there is variation in the orientation rate of the (222) plane of the PZT layer among the samples.
The reason for this is that when crystallization annealing treatment is performed, not only the (222) plane but also the (100) and (101) planes of the PZT layer appear. If stable orientation is not realized, the amount of switching electric charges QSW does not stabilize. As a result, the yield of ferroelectric capacitors or FeRAMs using such ferroelectric capacitors may fall.